1. Field of the Invention
The invention relates to a multi-part piston for an internal combustion engine, having an upper piston part that has a piston crown, and a lower piston part. Each of the piston parts has an inner and an outer support element, which elements delimit an outer circumferential cooling channel and an inner cooling chamber, whose cooling chamber bottom has an opening.
2. The Prior Art
A piston of this type is disclosed in European Patent No. EP 1 222 364 B1. The opening in the cooling chamber bottom allows cooling oil to flow away out of the inner cooling chamber in the direction of the piston crown, in order to achieve a cooling effect as a consequence of the oil passage from the outer circumferential cooling channel to the inner cooling chamber, and to lubricate the piston pin. In order to achieve this goal, the opening in the cooling chamber bottom cannot be too large, because then, the cooling oil would no longer flow away in a metered manner, and its cooling effect in the inner cooling chamber would at least be reduced. This means that the cooling chamber bottom is configured essentially as a relatively wide and thin circumferential ring land that extends approximately in the radial direction, in the upper region of the lower piston part. However, such a structure is difficult to produce. In the case of a forged lower piston part, in particular, there is the additional problem that the microstructure of the material is changed in the region of the ring land, as the result of forging, and this results in an increase in stress in the material structure.